1. Field of the Invention
The invention relates to non-stoichiometric substances and more particularly to nanostructured non-stoichiometric substances and products incorporating such substances.
In one aspect, the invention comprises a nanostructured filler, intimately mixed with a matrix to form a nanostructured composite. At least one of the nanostructured filler and the nanostructured composite has a desired material property which differs by at least 20% from the same material property for a micron-scale filler or a micron-scale composite, respectively. The desired material property is selected from the group consisting of refractive index, transparency to light, reflection characteristics, resistivity, permittivity, permeability, coercivity, B—H product, magnetic hysteresis, breakdown voltage, skin depth, curie temperature, dissipation factor, work function, band gap, electromagnetic shielding effectiveness, radiation hardness, chemical reactivity, thermal conductivity, temperature coefficient of an electrical property, voltage coefficient of an electrical property, thermal shock resistance, biocompatibility and wear rate.
The nanostructured filler may comprise one or more elements selected from the s, p, d, and f groups of the periodic table, or it may comprise a compound of one or more such elements with one or more suitable anions, such as aluminum, antimony, boron, bromine, carbon, chlorine, fluorine, germanium, hydrogen, indium, iodine, nickel, nitrogen, oxygen, phosphorus, selenium, silicon, sulfur, or tellurium. The matrix may be a polymer (e.g., poly(methyl methacrylate), poly(vinyl alcohol), polycarbonate, polyalkene, or polyaryl), a ceramic (e.g., zinc oxide, indium-tin oxide, hafnium carbide, or ferrite), or a metal (e.g., copper, tin, zinc, or iron). Loadings of the nanofiller may be as high as 95%, although loadings of 80% or less are preferred. The invention also comprises devices which incorporate the nanofiller (e.g., electrical, magnetic, optical, biomedical, and electrochemical devices).
Another aspect of the invention comprises a method of producing a composite, comprising blending a nanoscale filler with a matrix to form a nanostructured composite. Either the nanostructured filler or the composite itself differs substantially in a desired material property from a micron-scale filler or composite, respectively. The desired material property is selected from the group consisting of refractive index, transparency to light, reflection characteristics, resistivity, permittivity, permeability, coercivity, B—H product, magnetic hysteresis, breakdown voltage, skin depth, curie temperature, dissipation factor, work function, band gap, electromagnetic shielding effectiveness, radiation hardness, chemical reactivity, thermal conductivity, temperature coefficient of an electrical property, voltage coefficient of an electrical property, thermal shock resistance, biocompatibility, and wear rate. The loading of the filler does not exceed 95 volume percent, and loadings of 80 volume percent or less are preferred.
The composite may be formed by mixing a precursor of the matrix material with the nanofiller, and then processing the precursor to form a desired matrix material. For example, the nanofiller may be mixed with a monomer, which is then polymerized to form a polymer matrix composite. In another embodiment, the nanofiller may be mixed with a matrix powder composition and compacted to form a solid composite. In yet another embodiment, the matrix composition may be dissolved in a solvent and mixed with the nanofiller, and then the solvent may be removed to form a solid composite. In still another embodiment, the matrix may be a liquid or have liquid like properties.
Many nanofiller compositions are encompassed within the scope of the invention, including nanofillers comprising one or more elements selected from the group consisting of actinium, aluminum, arsenic, barium, beryllium, bismuth, cadmium, calcium, cerium, cesium, cobalt, copper, dysprosium, erbium, europium, gadolinium, gallium, gold, hafnium, hydrogen, indium, iridium, iron, lanthanum, lithium, magnesium, manganese, mendelevium, mercury, molybdenum, neodymium, neptunium, nickel, niobium, osmium, palladium, platinum, potassium, praseodymium, promethium, protactinium, rhenium, rubidium, scandium, silver, sodium, strontium, tantalum, terbium, thallium, thorium, tin, titanium, tungsten, vanadium, ytterbium, yttrium, zinc, and zirconium.
“Domain size” as that term is used herein, refers to the minimum dimension of a particular material morphology. In the case of powders, the domain size is the grain size. In the case of whiskers and fibers, the domain size is the diameter. In the case of plates and films, the domain size is the thickness.
As used herein, a “nanostructured powder” is one having a domain size of less than 100 nm, or alternatively, having a domain size sufficiently small that a selected material property is substantially different from that of a micron-scale powder, due to size confinement effects (e.g., the property may differ by 20% or more from the analogous property of the micron-scale material). Nanostructured powders often advantageously have sizes as small as 50 nm, 30 nm, or even smaller. Nanostructured powders may also be referred to as “nanopowders” or “nanofillers.” A nanostructured composite is a composite comprising a nanostructured phase dispersed in a matrix.
As it is used herein, the term “agglomerated” describes a powder in which at least some individual particles of the powder adhere to neighboring particles, primarily by electrostatic forces, and “aggregated” describes a powder in which at least some individual particles are chemically bonded to neighboring particles.
2. Relevant Background
Most compounds are prepared as stoichiometric compositions, and numerous methods of preparing substances for commercial use are motivated in objective to create stoichiometric compounds. For example, producers of titania fillers, copper oxide catalysts, titanate dielectrics, ferrite magnetics, carbide tooling products, tin oxide sensors, zinc sulfide phosphors, and gallium nitride electronics all seek stoichiometric compositions (TiO2, CuO, BaTiO3, NiFe2O4, TiC, SnO2, ZnS, and GaN, respectively).
Those skilled in the art will note that conventional powders of oxides and other compounds, when exposed to reducing atmospheres (e.g. hydrogen, forming gas, ammonia, and others) over a period of time, are transformed to non-stoichiometric materials. However, the time and cost of doing this is very high because the inherent diffusion coefficients and gas-solid transport phenomena are slow. This has made it difficult and uneconomical to prepare and commercially apply stable non-stoichiometric forms of materials to useful applications.
Limited benefits of non-stoichiometric materials have been taught by others; for example, Sukovich and Hutcheson in U.S. Pat. No. 5,798,198 teach a non-stoichiometric ferrite carrier. Similarly, Menu in U.S. Pat. No. 5,750,188 teaches a method of forming a thin film of non-stoichiometric luminescent zinc oxide. The film is a result of a thermodynamically favored defect structure involving non-stoichiometric compositions where the non-stoichiometric deviation is in parts per million.
A very wide variety of pure phase materials such as polymers are now readily available at low cost. However, low cost pure phase materials are somewhat limited in the achievable ranges of a number of properties, including, for example, electrical conductivity, magnetic permeability, dielectric constant, and thermal conductivity. In order to circumvent these limitations, it has become common to form composites, in which a matrix is blended with a filler material with desirable properties. Examples of these types of composites include the carbon black and ferrite mixed polymers that are used in toners, tires, electrical devices, and magnetic tapes.
The number of suitable filler materials for composites is large, but still limited. In particular, difficulties in fabrication of such composites often arise due to issues of interface stability between the filler and the matrix, and because of the difficulty of orienting and homogenizing filler material in the matrix. Some desirable properties of the matrix (e.g., rheology) may also be lost when certain fillers are added, particularly at the high loads required by many applications. The availability of new filler materials, particularly materials with novel properties, would significantly expand the scope of manufacturable composites of this type.